Method of making a heating element of moylbdenum silicide type and a heating element

ABSTRACT

A method for the production of a heating element that is composed essentially of molybdenum silicide and alloys of that basic material, and a heating element formed from such material. A material is produced that contains substantially Mo(Si 1-x Al x ) 2  and Al 2 O 3  by mixing a molybdenum aluminum silicide Mo(Si 1-y Al y ) 2  with bentonite clay in a known manner. The bentonite clay contains impure or contaminating substances with which molybdenum silicide cannot be alloyed and with which the symmetry of the crystal lattice of the molybdenum silicide is retained with a combined content of less than 2000 ppm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a heatingelement of the molybdenum silicide type and also to a heating element.

2. Description of the Related Art

An electric resistance element of the molybdenum silicide type isdescribed in Swedish Patent Specifications 0003512-1 and 0004329-9.According to patent specification 0003512-1 the resistance material ofthe heating element includes Mo(Si_(1-x)Al_(x))₂ which contains aluminumto an extent at which the formation of pest is essentially prevented.

It has been found that when such material is operated in a temperaturerange of 400-600° C. no pest, or only a slight amount of pest, isformed. Pest is formed by virtue of the formation of MoO₃ from MoSi₂ andO₂.

The reason why the formation of pest is significantly reduced or iseliminated is due the formation of Al₂O₃ on the surface of the element.

According to one preferred embodiment x is caused to lie in the range of0.2-0.6.

The other patent specification, 0004329-9, teaches a method ofincreasing the useful life span of heating elements that consist chieflyof molybdenum silicide and alloys of that basic material where theelement operates at high temperatures.

According to that patent specification, the heating element containsaluminum to an extent which is sufficient to maintain a stable, slowlygrowing layer of aluminum oxide on the surface of the heating element.

According to a preferred embodiment the heating element materialcontains Mo(Si_(1-x)Al_(x))₂, where x lies in the range of 0.2-0.6.

A material of the molybdenum silicide type that contains aluminum hasbeen found to possess improved corrosion properties at both low and hightemperatures.

Such material is often produced by mixing MoSi₂ powder with oxidic rawmaterial, such as aluminosilicates. When the raw material is bentoniteclay, there is obtained a relatively low melting point which contributestowards so-called smelt phase sintering, which results in densematerials that contain MoSi₂ and a proportion of aluminum silicatecorresponding to 15-20 percent by volume.

Bentonite clay has different compositions. Some bentonites include 60%by weight SiO₂ while some contain somewhat more than 70% by weight SiO₂.Although the Al₂O₃ content varies, it normally lies between 13-20% byweight. The melting point varies between about 1200-1400° C.

Bentonite clay that contains chiefly SiO₂ can be used in the productionof heating elements containing Mo(Si_(1-x)Al_(x))₂. When sintering withan Al-alloyed silicide there takes place a chemical exchange reaction inwhich the greater affinity of the oxygen to Al than to Si results in Sileaving the aluminum silicate and entering the silicide as a result ofAl leaving the silicide and being taken up by the oxide phase. Thatexchange reaction also contributes towards improved sintering propertiesof the composite material. The final material containsMo(Si_(1-x)Al_(x))₂ that is substantially depleted of Al, where theoxide phase contains Al₂O₃ in all essentials.

The standard procedure of manufacture involves mixing molybdenum,silicon, and aluminum in powder form and firing the powder mix normallyunder a shielding gas atmosphere. This results in a cake of the materialMo(Si_(1-y)Al_(y))₂, where y is larger than x as a result of saidexchange reaction. The reaction is exothermic. The cake is then crushedand ground down to a fine particle size normally in the order of 1-20μm. The resulting powder is mixed with bentonite clay to form a wetceramic material. The material is extruded and dried to a rod form whosediameter corresponds to the diameter of the subsequent heating element.The material is then sintered at a temperature that exceeds the meltingtemperature of the included components.

However, there is a drawback with an element of that kind. The problemis that the oxide that forms on the surface of the element, namelyAl₂O₃, sometimes peels away or flakes off, i.e., loosens from thesurface of the element, in cyclic operation.

A peeling oxide gives poorer protection against continued oxidation ofaluminum, which becomes impoverished in the outer surface of the elementmore quickly. Moreover, peeling oxide can contaminate the oven in whichthe element is fitted, with the risk that performance and the appearanceof products heat treated in ovens that have such elements will besignificantly impaired. This restricts the use of such elements inheating processes.

This problem is solved by the present invention.

SUMMARY OF THE INVENTION

The present invention thus relates to a method of producing a heatingelement that is composed substantially of material of the molybdenumsilicide type and alloys of that basic material. A material is producedthat contains chiefly Mo(Si_(1-x)Al_(x))₂ and Al₂O₃ by mixing molybdenumaluminosilicide (Mo(Si_(1-y)Al_(y))₂) with bentonite clay in a way knownper se. The bentonite clay includes impurities with which the molybdenumsilicide cannot be alloyed and with which the symmetry of the crystallattice of the molybdenum silicide will be retained, with a combinedcontent of 2000 ppm.

The present invention also relates to a heating element that issubstantially of the molybdenum silicide type and alloys of that basicmaterial, wherein the element is composed chiefly of the materialsMo(Si_(1-x)Al_(x))₂ and Al₂O₃, and wherein the material contains impuresubstances with which the molybdenum suicide can not be alloyed and withwhich the symmetry of the crystal lattice of the molybdenum silicide ismaintained with a combined content of less than 2000 ppm.

The invention will be described in more detail in the followingdescription.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the invention, a heating element that is composedchiefly of molybdenum silicide type material and alloys of that basicmaterial is produced in principle by the method described above. Thereis thus produced a powder that contains essentially Mo(Si_(1-y)Al_(y))₂and that is mixed with bentonite clay that contains Al₂O₃.

According to the invention, the bentonite clay includes impurities withwhich the molybdenum silicide cannot be alloyed, so that the symmetry ofthe crystal lattice will be retained, with a combined content of lessthan 2000 ppm. Bentonite clay that has those low impurity contents canbe produced by purifying the bentonite clay with the aid of knownchemical cleaning processes.

In distinction to that approach, it is possible to partly substitute formolybdenum Re or W in the material Mo(Si_(1-x)Al_(x))₂ without changingthe symmetry of the crystal lattice.

According to one preferred embodiment of the invention, the combinedcontent of the impurity substances Mg, Ca, Fe, Na, and K is below 2000ppm.

According to another preferred embodiment of the invention, the contentof said impure substances is below 1000 ppm.

It has been found, surprisingly, that there is obtained at such lowcontaminant contents an oxide which does not peel after cyclic operationbetween room temperature and high temperatures, for instance 1500° C.

According to one embodiment x lies in the range of 0.4-0.6.

According to one preferred embodiment x lies in the range of 0.45-0.55.

The present invention thus solves the problem mentioned in theintroduction and enables the present heating element to be usedbeneficially in ovens without detriment to the material treated in theoven.

The present invention shall not be considered to be limited to theabove-described embodiments since variations can be made within thescope of the accompanying claims.

1. A method of producing a heating element that is composed essentiallyof molybdenum silicide type material and alloys of that basic material,said method comprising the steps of: producing a heating elementmaterial that contains substantially Mo(Si_(1-x)Al_(x))₂ and Al₂O₃ bymixing a molybdenum aluminium silicide Mo(Si_(1-y)Al_(y))₂ withbentonite clay in a known manner, wherein the bentonite clay containsimpure substances with which molybdenum silicide cannot be alloyed andwith which the symmetry of the crystal lattice of the molybdenumsilicide is retained at a combined impure substance content of less than2000 ppm; and forming a heating element from the heating elementmaterial.
 2. A method according to claim 1, wherein the impuresubstances include at least one of Mg, Ca, Fe, Na, and K, and thecombined content of the impure substances is less than 2000 ppm.
 3. Amethod according to claim 2, wherein the content of said impuresubstances is caused to be less than 1000 ppm.
 4. A method according toclaim 1, wherein x lies in the range of 0.4-0.6.
 5. A method accordingto claim 1, wherein x lies in the range of 0.45-0.55.
 6. A methodaccording to claim 1, including the step of substituting molybdenumpartly with at least one of Re and W in the materialMo(Si_(1-x)Al_(x))₂.
 7. An electrical heating element that is composedsubstantially of the molybdenum silicide type material and alloys ofthat basic material, said element comprising the materialsMo(Si_(1-x)Al_(x))₂ and Al₂O₃; wherein the heating element materialcontains impure substances with which the molybdenum silicide can not bealloyed and with which the symmetry of the crystal lattice of themolybdenum suicide is maintained, wherein the impure substances arepresent at a combined content of less than 2000 ppm.
 8. A heatingelement according to claim 7, wherein the impure substances include Mg,Ca, Fe, Na, and K and the combined content of the impure substances isless than 2000 ppm.
 9. A heating element according to claim 8, whereinthe impure substance content of said material is less than 1000 ppm. 10.A heating element according to claim 7, wherein x lies in the range of0.4-0.6.
 11. A heating element according to claim 7, wherein x is causedto lie lies in the range of 0.45-0.55.
 12. A heating element accordingto claim 7, wherein molybdenum in the material Mo(Si_(1-x)Al_(x))₂ isreplaced partially with at least one of Re and W.